Conventional nozzles for continuous casting of steel are often made of alumina-graphite refractories. Nozzles for continuous casting include a long nozzle and an air seal pipe which are used between a ladle and a tundish and a submerged nozzle used between a tundish and a mold. These nozzles are strictly required to have corrosion resistance against molten steel or slag and spalling resistance in nature of the condition of their use. For the time being, alumina-graphite materials have been of wide use to cope with these requirements.
Where a nozzle made of alumina-graphite materials is used, particularly for casting aluminum killed steel having a high aluminum content, alumina (Al.sub.2 O.sub.3) resulting from oxidation of aluminum deposits on the inner wall of the nozzle to cause clogging.
Multiple continuous casting has recently been spreading for improving productivity. If a nozzle is clogged due to alumina deposition (adhesion) in multiple continuous casting, the flow of molten steel can no more be controlled, making it difficult to continue casting.
The clogged obstruction sometimes comes off the inner wall of the nozzle during casting. In this case, the obstruction enters the mold and is incorporated into cast steel to cause casting defects.
It seems that deposition of alumina onto a nozzle proceeds through the reactions between aluminum in molten steel and the refractories constituting a submerged nozzle, represented by the following reaction formulae. EQU (1) SiO.sub.2 (s)+C (s)=SiO (g)+CO (g) EQU (2) 3SiO (g)+2Al=Al.sub.2 O.sub.3 (s)+3Si EQU (3) 3CO (g)+2Al=Al.sub.2 O.sub.3 (s)+3C
At first, the reaction represented by formula (1) takes place between SiO.sub.2 (s) and C (s) present in the refractories to generate SiO (g) and CO (g). Subsequently, the reactions represented by formulae (2) and (3) occur between Al in molten steel and the produced SiO (g) and CO (g) to form Al.sub.2 O.sub.3 (s), which is deposited (adhered) on the surface of the inner wall of a nozzle. Alumina in molten steel is gradually accumulated on the thus produced alumina seed, finally blocking the nozzle.
Various methods have been studied and proposed as a means for preventing such nozzle clogging. For example, gas blowing is generally adopted as an effective means for preventing nozzle clogging.
Gas blowing is a method in which the inner wall of, e.g., a submerged nozzle is made of porous refractories, and gas (e.g., argon) is blown through the open pores to inhibit alumina from depositing on the inner wall. This method is effective on prevention of nozzle clogging and is adopted in many steel works.
However, while gas is made to flow in a sufficient amount enough to prevent nozzle clogging, fine gas bubbles tend to enter the mold to cause casting defects. Further, the gas causes the bath level of molten steel to vary considerably. As a result, the molten steel tends to take up inclusions which will cause defects of cast steel.
In addition to gas blowing, use of CaO-containing zirconia clinker is also known as a countermeasure to clogging as disclosed in JP-B-2-23494 (the term "JP-B" as used herein means an "examined published Japanese patent application"). According to this method, Al.sub.2 O.sub.3 particles precipitated in molten steel are reacted with CaO present in zirconia clinker to produce CaO--Al.sub.2 O.sub.3 -based low-melting compounds, which are removed along with the flow of molten steel, thereby to prevent alumina deposition.
The method of using zirconia clinker is considered effective on prevention of alumina deposition. In fact, a submerged nozzle whose inner wall is made up of a material containing CaO-containing zirconia clinker is used in a large number of continuous casting machines.
However, a nozzle using CaO-containing zirconia clinker is inferior in spalling resistance because CaO-containing zirconia clinker has a large thermal expansion coefficient and, being laid on the inner side of the nozzle, generates a great thermal stress on the outer side of the nozzle in the initial stage of casting.
On the other hand, JP-A-3-243258, JP-A-5-154628, JP-A-8-57601, and JP-A-8-57613 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") mention that clogging of a nozzle can be prevented by making the inner wall and other parts of the nozzle which come into contact with molten steel by using an oxide material having no or, if any, less than 1% by weight of, carbon. The publications describe that use of oxides, such as alumina or magnesia, at the parts coming into contact with molten steel is effective in preventing alumina deposition or carbon pickup.
However, since any of the materials used in the above-described methods contains substantially no carbon source, it necessarily has a high thermal expansion coefficient, resulting in poor spalling resistance.
Referring to the poor spalling resistance of these materials, JP-A-8-57601 and JP-A-3-243258 supra propose molding the inner wall part and other parts coming into contact with molten steel separately from the nozzle body of the nozzle and, after completing the nozzle body, laying the inner wall part, etc. by slip casting or injecting the oxide material or inserting a sleeve brick made of the oxide material. However, the separate molding method for producing a nozzle for continuous casting is very complicated, involving an increased number of steps and incurring a high cost of production.
JP-A-51-54836 also discloses a submerged nozzle whose inner wall part is made up of a carbon-free material. The material used here contains 90% by weight or more of SiO.sub.2 and therefore suffers a considerable corrosion in casting.
JP-A-63-203258 discloses a material having a carbon content of not more than 20% by weight. In the publication, however, no consideration is given to the grain size distribution of the raw materials used and the thickness of the inner wall part, and the material disclosed is unsatisfactory in thermal shock resistance.
Additionally, application of materials other than oxides to a nozzle is disclosed in JP-A-56-139260, in which a material containing 5% to 80% by weight of boron nitride is used.
In brief, conventional techniques for preventing clogging of a nozzle due to deposition of alumina include (1) gas blowing, (2) reaction between alumina in molten steel and the content (CaO content) in the nozzle material to form a low-melting compound, (3) molding the inner wall part by slip casting or injecting a carbon-free refractory material, but they have their several disadvantages.